Argon as the test of two interpretations of the LZ 248 keV recoil

Howard Baer; Vernon Barger

REVIEW

Argon as the test of two interpretations of the LZ 248 keV recoil

Howard Baer, Vernon Barger

High Energy Physics - Phenomenology · Submitted 14 Sep 2026

Pith's verdict: A clean, calculable test that would decide two interpretations of the LZ event—but it stands or falls on the assumption that the event is real.

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arXiv ID2609.15782 (version 2)

Pith IDJSD5I423, Pith's permanent record of this paper and its review

CitationHoward Baer, Vernon Barger. "Argon as the test of two interpretations of the LZ 248 keV recoil." arXiv:2609.15782 (2026). https://pith.science/paper/JSD5I423

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Editor's note

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Baer and Barger lay out a simple discriminative test. If the LZ 248 keV recoil is an exothermic downscatter, then existing DEAP-3600 data should contain an argon nuclear recoil line near 336 keV (for 200 GeV DM) at several events per tonne-year. The endothermic interpretation predicts nothing in argon. The kinematic calculation is straightforward, the rate ratio cancels most astrophysical uncertainties, and the paper clearly explains how the line position and rate depend only on kinematics and form factors. That is the paper's strength—it offers a concrete, testable prediction using data already on disk.

The soft spot is the load-bearing assumption: the entire argument treats the single LZ candidate as a real dark matter signal. The authors do not argue for this; they simply take it as given. If the event is a background fluctuation, the derived spliting δ becomes meaningless, and the false dichotomy between exothermic and endothermic interpretations loses its empirical foundation. The argon test would still be a valid test of exothermic models, but it would no longer 'decide what the LZ event is' because one of the interpretations would have no motivating event. A brief discussion of this conditional—or at least an explicit statment that the test's relevance depends on the LZ event being signal—would strengthen the paper.

Despite this, the paper is well-written, the arithmetic is correct, and it provides a falsifiable prediction. It belongs in the conversation around the LZ candidate. I would send it to a good referee; the right reader is anyone interested in whether the exothermic or endothermic interpretation can be distinguished with existing data. A minor revision that acknowledges the conditional nature would be appropriate.

Referee report

Major objections (3)

[Introduction and summary] The entire argument presupposes that the LZ 248 keV event is a genuine dark matter signal rather than a background fluctuation. The paper does not compute the probability that such an event could arise from backgrounds in the LZ data, nor does it cite such an analysis. If the event is later shown to be a background, the dichotomy between exothermic and endothermic interpretations loses its empirical motivation, and the argon test, while still valid as a test of exothermic models, no longer 'decides what the LZ event is'. The authors should explicitly state this assumption and discuss how the argon results would be interpreted if the LZ candidate is not a signal.
[The argon line, The data in hand] The rate calculation uses the standard halo model (v0 = 238 km/s, vesc = 544 km/s) and a Maxwellian velocity distribution. The cancellation of halo parameters in the ratio is robust only if the velocity distribution is similar for xenon and argon, which it is for a single smooth halo. However, the paper does not discuss the sensitivity of the prediction to non-Maxwellian features (e.g., a dark disk or a tidal stream) that could affect the exothermic line shape and the threshold for the endothermic case. A brief comment on this systematics would strengthen the argument.
[Table I and Fig. 1] The paper adopts a single argon quenching factor Leff = 0.25 from Gastler et al. (2012). More recent measurements (e.g., from DEAP or DarkSide) may have updated values or energy-dependent corrections. The line position in keVee depends linearly on Leff; a 10% systematic uncertainty shifts the expected photoelectron window by ~10%. The authors should note the source of the quenching value and the impact of a range of plausible values on the detectability.

Minor objections (5)

[Introduction] Reference [24] (DEAP 231-day search) is from 2019. There may be more recent results or reanalyses of the full first run; the paper should check whether the quoted exposure of 813 live days [25] has been published as a dark matter search.
[Table II] The third-run exposure of '~300 live days' is stated as an assumption without a source. Please indicate whether this is a publicly announced plan or a speaker's estimate, or remove it to avoid speculation.
[Figure 2] The caption says 'Best viewed in colour'—this is unnecessary for a printed journal, but acceptable if the journal allows it.
[General] The paper uses 'sec' as an abbreviation for 'section' in the text (e.g., 'Sec. III.A'). This is a common but informal shorthand; consider spelling out 'Section' for consistency with journal style.
[Equation (1)] The absolute value in the numerator of v_min is not strictly needed because the exothermic case has a negative δ_eff (or the definition of δ can be taken as positive). Clarify the sign convention for δ in the exothermic case.

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The paper

The paper claims

Existing argon data can decide the nature of the LZ 248 keV recoil event, distinguishing exothermic from endothermic dark matter interpretations.

The reading

The LUX-ZEPLIN (LZ) experiment detected a single nuclear recoil at 248 keV with no accompanying low-energy events. Two classes of dark matter models can explain this: exothermic (excited state decays to lighter state, releasing energy) and endothermic (ground state absorbs energy from the fastest halo particles). The paper shows that these two interpretations make opposite predictions for argon detectors. Exothermic models predict a nuclear-recoil line in argon at about 336 keV (for a 200 GeV dark matter particle) with a rate roughly ten times that in xenon. Endothermic models predict zero events because the required threshold velocity for argon exceeds the galactic escape speed. Data already recorded by the DEAP-3600 experiment contain enough exposure to detect tens of predicted exothermic events. A reanalysis of this existing data can settle the interpretation without building new detectors.

What carries the argument

The central mechanism is inelastic dark matter scattering kinematics. For exothermic scattering (downward transition releasing splitting $|\delta|$), the minimum velocity $v_{\rm min}$ vanishes when the recoil energy $E_R = |\delta| \mu_A/m_A$, producing a kinematical line broadened by the halo. For endothermic scattering (upward transition absorbing $\delta$), a threshold velocity $v_\star = \sqrt{2\delta/\mu_A}$ exists; for the splitting required by the LZ event ($\delta \sim 300$ keV), $v_\star$ on argon exceeds the galactic escape velocity. The Helm nuclear form factor is the second key: argon at 336 keV lies in its first diffraction lobe ($F^2\approx0.056$), while xenon at 248 keV lies near a second-lobe minimum ($F^2\approx1.6\times10^{-4}$), giving argon a large rate advantage.

Extended reading notes

Core claim

The single 248 keV nuclear recoil observed by LZ can be interpreted either as exothermic downscattering of a pseudo-Dirac dark matter state or as endothermic upscattering of a ground-state Higgsino. These two classes yield opposite predictions for argon targets: exothermic gives a spectral line at $E_{\rm Ar} = |\delta| m_\chi/(m_\chi+m_{\rm Ar})$, equal to 336 keV for $m_\chi=200$ GeV, at a rate per tonne-year that is about ten times the xenon rate that produced the LZ event; endothermic gives no events because the kinematics require a velocity above the halo escape velocity. The ratio depends only on two-body kinematics, nuclear form factors, and the charge structure of the coupling, not on mediator details or the excited fraction. The DEAP-3600 data from 2016 to 2020 contain 7.3 tonne·years of exposure, enough to see about 25 expected exothermic events, and can discriminate decisively.

Load-bearing premise

The argument rests on assuming that the single 248 keV recoil event in LZ is a genuine dark matter signal rather than a background fluctuation; if the event is a fluke, the entire test of exothermic vs. endothermic interpretations collapses.

Editorial extensions

If this is right

If the exothermic interpretation is correct, the DEAP-3600 data (first run of 813 live days) should contain about 6 events within the published 824 kg fiducial mass, and about 25 events in the full 3279 kg target.
Observation of the argon line would determine the dark matter mass from the two line positions (xenon and argon), which a single target cannot do.
The rate ratio cancels astrophysical and particle physics uncertainties ($\rho_\chi$, $f_2$, $\sigma_p$), providing a pure test of the scattering model class.
Detection of the predicted line would also confirm that the interaction is coherent and spin-independent, ruling out dominant spin-dependent couplings.
Non-observation of argon events at the predicted level would rule out all exothermic models for the LZ event, independently of the mediator or the origin of the splitting.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

A reanalysis of DEAP-3600 data for nuclear recoils between 200 and 600 keV (50–150 keV electron-equivalent) could set competitive cross-section limits on exothermic dark matter even if no signal appears, complementing xenon-based bounds.
If the argon line is found, combining DEAP-3600 and future DarkSide-20k data would enable a precise measurement of the line position and therefore the dark matter mass, with statistical precision of a few percent.
The test is universal for all exothermic models with a kinetically mixed dark photon mediator, because the line energy depends only on mass and splitting, making it a robust signature independent of the coupling strength or mediator mass.
A null result in argon would strengthen the case for endothermic Higgsino interpretations, which would then face their own naturalness and solar-capture constraints—an implication that the paper leaves implicit.

Original abstract

Argon decides what the LZ 248 keV recoil is. Read as an excited pseudo-Dirac state dropping to its ground state, the event predicts a nuclear-recoil line in argon at |\delta|\,m_\chi/(m_\chi+m_{\rm Ar}), 336 keV at 200 GeV, at ten times the xenon rate, and the two line positions fix the mass. Read as a ground-state Higgsino lifted by the fastest halo particles, it predicts no argon events, since the argon threshold exceeds the halo maximum. The ratio depends only on kinematics and form factors, and the DEAP-3600 data already recorded hold tens of expected events.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 3,694 input tokens.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.17 · human participants 0.02 · controlled comparison 0.14 · randomized assignment 0.01 · states quantitative result 0.91 · compares with prior results 0.77 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v2. Results and the main claim are underlined; show every role.

Argon decides what the LZ 248 keV recoil is. Read as an excited pseudo-Dirac state dropping to its ground state, the event predicts a nuclear-recoil line in argon at |\delta|\,m_\chi/(m_\chi+m_{\rm Ar}), 336 keV at 200 GeV, at ten times the xenon rate, and the two line positions fix the mass. Read as a ground-state Higgsino lifted by the fastest halo particles, it predicts no argon events, since the argon threshold exceeds the halo maximum. The ratio depends only on kinematics and form factors, and the DEAP-3600 data already recorded hold tens of expected events.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#	sentence	role	the authors' own work	states the main conclusion	strength
s0	Argon decides what the LZ 248 keV recoil is.	contribution 0.49	0.14	0.50	asserted 1.00
s1	Read as an excited pseudo-Dirac state dropping to its ground state, the event predicts a nuclear-recoil line in argon at \|\delta\|\,m_\chi/(m_\chi+m_{\rm Ar}), 336 keV at 200 GeV, at ten times the xenon rate, and the two line positions fix the mass.	interpretation 0.76	0.39	0.21	asserted 0.94
s2	Read as a ground-state Higgsino lifted by the fastest halo particles, it predicts no argon events, since the argon threshold exceeds the halo maximum.	interpretation 0.75	0.32	0.14	asserted 0.99
s3	The ratio depends only on kinematics and form factors, and the DEAP-3600 data already recorded hold tens of expected events.	result 0.77	0.20	0.17	asserted 0.97

What kind of paper the classifier reports this is: theoretical result 0.78 · evidence from analysis of existing data 0.77.

What was examined, and what was not

sources read

arXiv listing abstract v2

not examined

full text
figures and tables
references and citing sentences
relations to other papers

levels run

P abstract

not run

S sections
T body sentences
C citations
R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures

**Figure 1.** FIG. 1. What argon should see if the LZ recoil is a downscat

**Figure 2.** FIG. 2. Why argon wins. Helm form factors of argon (solid)

Figures reproduced from arXiv:2609.15782 by the authors.

References

Forward citations

Cited by 16 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

The Electron-Recoil Response Tail and the LZ High-Energy Event Background
hep-ex 2026-10 accept novelty 7.0 of 10

The published LZ electron-recoil response characterization does not determine the tail probability needed for the background estimate; tiny quantile shifts (0.14%) can change the ER contribution by a factor of 13.
The LZ event with a two-state dark matter halo
hep-ph 2026-09 conditional novelty 7.0 of 10

In a two-state pseudodirac dark matter halo, exothermic scattering from the heavier state dominates the LZ 248 keV candidate above a small excited-state fraction, lowering the required cross section dramatically and r...
Heavy Pseudo-Dirac Doublet Dark Matter and Non-Thermal Leptogenesis in Light of the LZ Event
hep-ph 2026-10 conditional novelty 6.0 of 10

Heavy pseudo-Dirac doublet dark matter and non-thermal leptogenesis can be reconciled in a low-reheating inflaton-domination cosmology only with kinematically forbidden dark matter pair emission and fine-tuned seesaw ...
Exothermic Dark Matter with a Dark Higgs: The LZ 248 keV Event and the Galactic Center Excess
hep-ph 2026-10 conditional novelty 6.0 of 10

Exothermic dark matter in a dark U(1) model with a dark Higgs can simultaneously explain the LZ 248 keV event and the Galactic Center gamma-ray excess.
Testing the inelastic dark matter interpretation of the LZ event with heavy-element experiments
hep-ph 2026-10 conditional novelty 6.0 of 10

The thermal Higgsino dark matter model can fit the LZ 248 keV event only if the dark matter velocity distribution has an extreme high-velocity tail, and heavy-element experiments can test this scenario.

Show 11 more citing papers

Exothermic dark-photon dark matter and the LZ high-recoil event: finite-reheating constraints
hep-ph 2026-10 accept novelty 6.0 of 10

该论文通过有限再加热的冻结产生机制，在45 GeV激发暗光子暗物质模型下，计算了LZ实验248 keV反冲候选事件所需的再加热温度。
Decaying Dark Matter for the Exothermic Recoil at LZ
hep-ph 2026-09 conditional novelty 6.0 of 10

Decay regeneration of a GeV-scale excited dark state produces the LZ 248 keV recoil as an exothermic downscattering line and predicts argon lines at 700 to 810 keV.
The high-velocity dark matter halo of the Milky Way in light of the LZ 248 keV event
hep-ph 2026-09 accept novelty 6.0 of 10

Using Milky Way analogues from TNG-50 and FIRE-2, the maximum laboratory-frame dark matter speed on June 16 is 793 km/s with 95% containment from 697 to 876 km/s, and the co-rotation of the dark matter halo does not l...
Probing LZ-Motivated TeV Higgsino Dark Matter through Heavier Sparticles at LHC
hep-ph 2026-10 accept novelty 5.0 of 10

对于LZ-动机的1.1 TeV Higgsino暗物质，当前LHC Run-2排除gluino至2.21 TeV，HL-LHC可达2.45-2.60 TeV排除和2.26-2.37 TeV发现，而squark、stop、sbottom等通道灵敏度不足。
Endothermic Dark Matter at LZ from a Decaying Parent
hep-ph 2026-09 conditional novelty 5.0 of 10

A decaying dark parent can supply both the energy and the flux for an endothermic dark matter interpretation of the LZ 248 keV nuclear recoil candidate, fixing the required lifetime at about 4.4 x 10^20 seconds.
Environment-dependent mass splitting to suppress solar capture in the inelastic-doublet interpretation of the LZ event
hep-ph 2026-09 conditional novelty 5.0 of 10

A density-dependent scalar field value is shown to enlarge the inelastic dark matter mass splitting inside the Sun, suppressing solar capture while keeping the laboratory value at 350 keV, at the price of severe fine-tuning.
The LHC is not enough: the LZ High-Recoil Event at FCC-hh and a Muon Collider
hep-ph 2026-09 conditional novelty 5.0 of 10

The LZ 248 keV recoil, read as inelastic electroweak dark matter, is beyond HL-LHC reach but coverable by FCC-hh and a multi-TeV muon collider.
LUX-ZEPLIN's Stairway to Hea$\nu$en: limits on elastic scatters of dark matter from solar capture
hep-ph 2026-09 conditional novelty 5.0 of 10

Using solar capture and neutrino telescope data, the authors rule out broad regions of parameter space for elastic dark matter models that could explain the LUX-ZEPLIN 248 keV recoil event.
A scalar-extended ${\rm U(1)_{L_{\mu}-L_{\tau}}}$ explanation of the LUX-ZEPLIN 248 keV excess
hep-ph 2026-10 conditional novelty 4.0 of 10

A scalar-extended U(1)Lmu-Ltau model with a roughly 100 keV dark matter mass splitting reproduces the LZ 248 keV recoil for TeV-scale dark matter while passing relic, muon g-2, trident, and collider constraints.
Inelastic Dark Matter from a Higher-Dimensional Operator: The LUX-ZEPLIN High-Recoil Event and Thermal Relic
hep-ph 2026-09 conditional novelty 4.0 of 10

A single dimension-6 Z-portal operator can simultaneously explain the 248 keV LZ recoil event via inelastic dark matter and the observed thermal relic density for a dark matter mass of roughly 0.2-1 TeV.
Inelastic dark matter mediated by natural dark photon
hep-ph 2026-09 reject novelty 4.0 of 10

A Z2-breaking dark sector with two nearly degenerate scalar dark matter states and a GeV-scale, weakly mixed dark photon is proposed as an exothermic inelastic explanation of the LZ 248 keV event.

Discussion

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